What is the key difference between primary and secondary sewage treatment?

Hint: The process of removal of contaminants from municipal wastewater and industrial wastewater is called sewage treatment. In addition, depending upon the type of removal of contaminants, it is categorized as primary or secondary sewage treatment.

Complete answer: The process which involves the removal of contaminants from municipal wastewater, and industrial wastewater is called sewage treatment. Removal of contaminants takes place through physical, chemical, and biological processes. After undergoing these processes the treated wastewater or effluent is safe enough to release into the environment. Sewage treatment is also referred to as wastewater treatment. The whole process includes five steps: pre-treatment, primary treatment, secondary treatment, tertiary treatment and fourth treatment stage. A physical process which involves the removal of large impurities from wastewater is termed as primary treatment. Heavy solid waste is allowed to settle at the bottom whereas oil, grease, and lighter solid waste float on the surface in primary treatment. After removal of settled and floating materials, the remaining liquid waste may be discharged or subjected to secondary treatment. Dissolved and suspended biological matter are removed in secondary treatment. Water-borne microorganisms are responsible for the process of secondary treatment. So, the main difference between primary and secondary treatment is that primary treatment is a physical process in which large impurities are removed whereas secondary treatment is a biological process performed using microbes.

Note: Decrease in the nutrient levels of water where the treated sewage flows is the disadvantage of sewage treatment plants. It is possible to reuse sewage effluent for drinking water with the use of suitable technology. This is done in places where there is a shortage of water, for example, Singapore and Windhoek.